Research leading to the completion and reduction to practice of this invention was supported, in part, by the Florida Department of Transportation via Contract No. 3366.
1. Field of the Invention
The invention relates to a method and system for determining the permeability to water of concrete.
2. Discussion of the Prior Art
Reinforced and pre-stressed concrete structures are commonly used in the construction of roadways, bridges, buildings, pre-cast concrete elements and the like. Such concrete structures generally comprise reinforced steel embedments such as rods, mesh or cables which are enclosed in formed concrete. It has been found that such concrete structures tend to deteriorate, with cracking and spalling of the concrete, when corrosive ions permeate the concrete and cause the reinforcing steel to corrode. Ions such as chlorides are likely to be present where road de-icing salts are used, and in marine environments. The chloride ions tend to depassify the alkali components of the concrete, and cause corrosion of the reinforcing steel embedments. The corrosion products of the reinforcing steel have a larger volume than the original steel and, therefore, create internal pressures in the concrete structure that causes it to crack and spall.
A related problem is the carbonation of the concrete structure. Carbonation occurs when carbon dioxide and carbon monoxide, from the exhaust of an internal combustion engine or the atmosphere, permeate the concrete and react with the hydrated cement compounds forming calcium carbonate. This tends to neutralize the alkaline environment surrounding the steel embedment, making the steel vulnerable to corrosion if moisture and oxygen are available.
Unwanted excessive permeability can occur in all concrete because of poor mix design, e.g., excessive water content, lack of fines or excessive air entrapment, or because of inadequate or inconsistent consolidation or due to poor finishing of the concrete. In addition to being susceptible to corrosion of the steel embedments, concrete having a high permeability tends to have a lower compressive and tensile strength and lessened durability and abrasion resistance than concretes of low permeability.
Corrosion problems also arise because of water and chloride ions leakage through joints in the concrete. Leakage through joints, as opposed to permeation through the concrete matrix, can occur in caulked joints such as flexible joints located between slabs of concrete. Leakage can also occur in cold joints which are joints between portions of a concrete structure which are cast at different times so that the concrete matrix loses continuity. Another location where water leakage problems frequently arise is in anchors for post-tensioning tendons which are typically sealed with a cement mortar patch after post-tensioning.
Sealers such as silane and siloxane solutions are often applied to the concrete and masonry to reduce the permeability and leakage of the structures. However, sealers tend to erode with time, thus reducing their effectiveness as barriers to ion intrusion. In addition, a sealer may not be properly or evenly applied to a structure so that the protection against weathering and ion intrusion varies over the surface of the structure.
Concrete formulations which have low absorption and low permeability have also been developed; however, these formulations are dependent on proper formulation, installation and curing for their effectiveness, and it is desirable to have quality control mechanisms even when these concrete formulations are employed.
In evaluating the permeability characteristics of concrete and masonry, there are three main elements which determine the overall permeability of the structure. These are: (1) leakage through joints and cracks, (2) surface permeability, and (3) matrix permeability. Where there are joints and cracks in the structure, these may be the major sources of liquid and ion penetration into the structure and may dominate any measure of the overall permeability of a structure.
In continuous, undamaged concrete, the surface permeability may be the limiting factor in the overall liquid and ion permeability, for example, in a low density concrete having a properly applied sealer. In other concrete structures, the matrix permeability may be the limiting factor, for example, in a high density concrete with a poor finish.
Several in situ tests of permeability have been proposed. U.S. Pat. No. 3,184,957 discloses an apparatus which comprises a series of reservoirs which, by the displacement of liquid from one reservoir to another, provide a stream of air or vapor to a dome placed on asphalt paving being tested. The degree of permeability is determined by measuring the time required to displace a fixed volume of liquid or the amount of liquid displaced in a fixed time. In an alternative embodiment, a slight vacuum is provided in the dome by the displacement of liquid.
U.S. Pat. No. 4,531,403 describes a system involving the use of a straddle packer and the application of negative fluid pressure to a sealed area to test for cracks and faults in structures such as mine roofs and the like.
U.S Pat. No. 3,861,196 discloses an apparatus having a central liquid chamber having an open end surrounded by an annular chamber in which a pressurized bladder seals the central chamber. The chambers are placed against the structure to be tested, and a liquid is provided under pressure to the central chamber. The flow rate of the liquid into the central chamber provides a measure of the permeability of the structure.
U.S. Pat. No. 4,979,390 describes a system for measuring the surface permeability of concrete comprising an apparatus for sealing a head chamber against the surface of a concrete structure, means for applying a partial vacuum to the head chamber and a pressure gauge for measuring pressure differences in the head chamber.
A report by the Concrete Society entitled "Permeability Testing of Site Concrete--A Review of Methods and Experience" describe the Figg method of determining permeability in which a hypodermic needle is sealed into a hole in a concrete structure and a vacuum is provided in the needle. The pressure increase is measured to provide a measure of the air permeability of the concrete. The Concrete Society report also describes the initial surface absorption test (ISAT) in which liquid absorption of concrete is tested using a cap which is mechanically attached to the surface of a structure and into which water is fed from a reservoir. These test apparatii and methods suffer from the need for mechanical attachment such as bolting or gluing of the test equipment to the structure so as to obtain a good seal between the cap and the concrete.
A paper by K. Schonlin and H.K. Hilsdorf entitled "Permeability as a Measure of Potential Durability of Concrete--Development of a Suitable Test Apparatus" (undated) describes a test apparatus for testing permeability of prepared samples of concrete in the laboratory. A concrete disk is cast in an air-tight rubber ring. A test apparatus, having a vacuum pump connected to a vacuum chamber located over the concrete disk, measures permeability based on the pressure increase in the vacuum chamber after the vacuum pump is isolated from the vacuum chamber.
It is desirable to provide an in situ method and apparatus for testing the permeability of the surface of concrete structures which, in addition to testing permeability, allows the tester to identify flaws in the structure which contribute to the permeability so that appropriate repairs may be made as necessary. In addition, it is desirable to provide a method and apparatus which is easily portable and which may be rapidly set up for testing. In addition, it is desirable to provide an apparatus and method which easily provides consistent measurements of permeability, allowing the development of a consistent index of permeability with which to compare structures and sealers at any time. It is preferable that such a method and apparatus be adapted to provide a quality control check so that a structure may be tested as it is erected.